Hybrid drives are known since a long time. Typically, they contain an internal combustion engine and at least one electric motor, which work in series or parallel on one driving shaft. For the coupling of the drive engines, a gearbox arrangement is necessary, mostly a superimposition gear and a cross gear set.
From U.S. Pat. No. 3,566,717, the entire contents of which is incorporated herein by reference, a drive system with an internal combustion engine as primary driving engine, and a gearbox with electric power branching is known. The signal preset by the operator is analysed as a desired torque on the driving wheel. The electric engine is controlled such that the electric engine runs with constant rotational speed even at variable load.
From EP 0 867 323, the entire contents of which is incorporated herein by reference, a method for the operation of a hybrid drive system has become known, in which the pedal position preset by the driver is interpreted as desired power on the driving wheel. The desired wheel torque is determined from a characteristic diagram, as a function of the pedal value (driver's desired torque) and of the actual travelling speed. With the actual travelling speed, the desired driving power is determined from the desired wheel torque. A consumption-favourable combination of torque and rotational speed, which satisfies the desired driving power, is read out from a previously fixed torque/rotational speed characteristic curve of the internal combustion engine. In this method, it is disadvantageous that the driver has to control the travelling speed himself, by presetting the torque or the power on the wheel, respectively.
The present invention is based on the objective to provide an apparatus for controlling a hybrid drive system for a motor vehicle, an industrial truck in particular, which minimises the fuel consumption and provides that internal combustion engine and electric engine are only charged with permitted torques. In addition, the travelling behaviour of the vehicle is to be improved,